Method of fabricating a saw device

ABSTRACT

A surface-acoustic-wave device comprises a substrate of a piezoelectric material for providing a passage of surface acoustic waves at an upper major surface of the substrate, a plurality of interdigital electrodes provided on the piezoelectric substrate in a row in correspondence to the passage of surface acoustic waves such that the electrodes are aligned in the propagating direction of the surface acoustic waves, each of the plurality of interdigital electrodes having a first part connected to a bonding pad on the substrate for external electric connection and having a plurality of finger electrodes extending parallel with each other in a direction crossing the propagating surface acoustic waves, and a second part separated from the first part and having a plurality of finger electrodes extending parallel with each other in a direction opposite to the direction of the finger electrodes of the first part. An interconnection pattern is provided on the surface of the substrate in correspondence to a region offset from the passage of the surface acoustic waves for connecting the second parts of adjacent interdigital electrodes with each other.

This application is a division of application Ser. No. 07/683,042, filedApr. 10, 1991, now U.S. Pat. No 5,243,249.

BACKGROUND OF THE INVENTION

The present invention generally relates to a so-calledsurface-acoustic-wave device having a plurality of interdigitalelectrodes, and in particular to the structure and fabrication processfor eliminating the occurrence of sparking discharges between thefingers of the electrodes during the process of device fabrication.

Recently, the demand for increased operational speed of informationprocessing apparatuses and communication apparatuses has caused theshift of the frequency of for the carriers or signals to higherfrequency regions. In correspondence to such a shift of the frequency,filters capable of operating in such high frequency regions arerequired. For this purpose, the surface-acoustic-wave (abbreviatedhereinafter as SAW) devices are used.

In view of the expected developments in the future, Particularly in thefield of automobile telephones and portable telephones, efforts arebeing made to develop SAW devices having a sharp attenuation in thefrequency region outside the pass-band while maintain a uniformband-pass characteristic. By using the SAW devices in place of theconventional dielectric filters, the size of the filters can be reducedto about 1/30th of the size of conventional filters and the size of thetelephone can be reduced accordingly.

A typical SAW device, such as the SAW filter, uses a piezoelectricsubstrate having large electromechanical coupling coefficients and smalltemperature coefficient of frequency. For example, a single crystal ofLiTaO₃ is used widely. The crystal of LiTaO₃ is cut in a predeterminedorientation, and interdigital electrodes are provided on the substrateas the input and output electrodes.

FIG. 1 shows the geometrical parameters characterizing a typicalinterdigital electrode.

Referring to FIG. 1, the electrode comprises a first part EL1 and asecond part EL2 each respectively having a number of fingers f₁ -f_(n)and g₁ -g_(n), wherein each finger has a width W and is separated fromadjacent fingers by a separation S. Designating the wavelength of thesurface acoustic wave as λ, the width W and the separation S aregenerally set to satisfy the relation W=S=λ/4. Thereby, the pitchdefined in FIG. 1 as P is set to P=λ/2. Further, each finger in theelectrode EL1 and each finger in the electrode EL2 are provided to forma uniform overlap as shown in FIG. 1. Such an electrode is called theuniform overlap electrode.

When forming a SAW filter having a central band pass frequency of 835MHz, for example, the pitch P is set to 2.45 μm while the width W andthe separation S are set to 1.23 μm in correspondence to the velocity of4090 m/sec of the surface acoustic wave in the X-direction. It should benoted that the foregoing velocity provides the wavelength λ of 4.9 μmfor the surface acoustic wave of 835 MHz. Generally, a pair of suchelectrodes EL1 and EL2 are provided. In the Particular applications ofSAW devices such as automobile telephones or portable telephones, on theother hand, devices having a small insertion loss, a wide Pass-band anda large suppression for the frequency components outside the pass band,are required. For example, an insertion loss of 3-5 dB or less, a passband of 25 MHz or more a side lobe suppression of 24-25 dB or more maybe required for the SAW filter having the central frequency of 835 MHz.

In order to satisfy these various requirements, the applicants of thepresent invention have proposed devices and previously disclosed in theUnited States, European, Korean and Canadian patent applicationsentitled "SURFACE-ACOUSTIC-WAVE FILTER HAVING A PLURALITY OFELECTRODES," based upon the Japanese patent applications 2-69121 and2-86236, and which are incorporated herein by reference.

FIG. 2 shows a wafer 1 on which a number of SAW devices 2 are formed.Typically, the SAW devices 2 are arranged in the rows and columns withdicing lines 3 formed between adjacent SAW devices 2. When the formationof the SAW devices 2 is completed, the wafer 1 is subjected to a dicingProcess wherein the wafer 1 is cut by a diamond saw along the dicinglines 3. Thereby, the SAW devices 2 are separated from each other.

FIG. 3 shows a typical conventional example of the SAW device 2 beforethe dicing process is carried out.

Referring to FIG. 3, the device 2 is surrounded by the dicing line 3,and a number of input electrodes 21 and a number of output electrodes 22are formed alternately on the surface of the substrate 1 to form a rowof electrodes. Each of the input electrodes 21 comprises a first part,corresponding to the part EL1 of FIG. 1, connected to a common inputbonding pad 28 and a second part, corresponding to the part EL2 of FIG.1, connected to a ground pad 27. The ground pad 27 is provided incorrespondence to each interdigital electrode 21. Similarly, each outputelectrode 22 comprises a first part connected to a common output bondingpad 28' and a second part connected a ground pad 27'. Further, a pair ofreflectors 25 of the open strip type are formed at both sides of the rowof electrodes 21 and 22. The dicing line 3 is merely a hypothetical linefor dicing the wafer into the individual devices.

Meanwhile, during the fabrication of the conventional SAW devices, therearises a problem in that, associated with various heating processesemployed during the fabrication, the surface of the Piezoelectricsubstrate is charged due to the pyroelectricity. It should be notedthat, in the structure of FIG. 3, the input electrodes 21 and the outputelectrodes 22 are isolated from each other. Further, in each input andoutput electrodes, the second part connected to the ground pad 27 or 27'is isolated from each other. In such a structure, the electric chargesinduced by the pyroelectric effect are accumulated and such accumulationof electric charges induces a sparking discharge between adjacentfingers of the interdigital electrodes. As the separation between thefingers is in the order of several microns in correspondence to thewavelength of the surface acoustic waves, a small amount of suchelectric charges is sufficient to cause such sparking.

FIG. 4 shows an electrode wherein such sparking has occurred. It shouldbe noted that such sparking discharge damages the finger of theinterdigital electrodes and hence the SAW device. In FIG. 4, the fingers50a and 50b represent the fingers that have experienced the sparkingdischarge. Thereby, the yield of the device is inevitably deteriorated.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful SAW device and a fabrication process thereof, whereinthe foregoing problems are eliminated.

Another and more specific object of the present invention is to providea SAW device and a fabrication process thereof, wherein the sparkingdischarge between the fingers of the electrode is eliminated.

Another object of the present invention is to provide a SAW device and afabrication process thereof, wherein the sparking discharge between thefingers of the electrode is eliminated while not degrading theoperational characteristics of the device.

Another object of the present invention is to provide a SAW device and afabrication process thereof, wherein a plurality of interdigitalelectrodes are formed on a piezoelectric substrate, and the interdigitalelectrodes are formed such that each finger of each interdigitalelectrode is connected with each other commonly, prior to the dicingprocess. According to the present invention, the problem of sparkingdischarge between the finger electrodes during the fabrication process,due to the pyroelectricity, is positively eliminated by neutralizing theelectric potential induced in various conductor parts of the device.

Another object of the present invention is to provide a SAW device and afabrication process thereof, wherein a plurality of interdigitalelectrodes are formed on a piezoelectric substrate, the interdigitalelectrodes being formed such that each finger of each interdigitalelectrode is connected commonly with each other by an interconnectionpattern prior to the dicing process, in which the interconnectionpattern is formed to avoid the path of the surface acoustic waves in thedevice. According to the present invention, undesirable deterioration ofthe device characteristics by the interconnection pattern disturbing thepassage of the surface acoustic waves is avoided.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a diagram showing a typical interdigital electrode;

FIG. 2 is a diagram showing a wafer on which a number of SAW devices isformed;

FIG. 3 is a diagram showing the construction of a SAW device formed onthe wafer of FIG. 2;

FIG. 4 is a diagram showing a example in which sparking dischargeoccurred in the SAW device of FIG. 3 during the fabrication processthereof;

FIG. 5 is a diagram showing the wafer carrying the SAW devices accordingto a first embodiment of the present invention;

FIG. 6 is a diagram showing the details of the SAW device of the firstembodiment of the present invention;

FIG. 7 is a diagram showing a SAW device according to a secondembodiment of the present invention;

FIG. 8 is a diagram showing a SAW device presented as a reference forshowing the effect of the present invention;

FIG. 9 is a diagram showing the frequency characteristic the SAW deviceof FIG. 7 in comparison with the characteristics of the device of FIG.8; and

FIG. 10 is a diagram showing a third embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 5 shows a wafer 51 on which a number of SAW devices 52 according toa first embodiment of the present invention are formed. The wafer 51comprises a piezoelectric single crystal such as 36°Y-X LiTaO₃ and mayhave a diameter of 51 cm and a thickness of 0.5 mm. The SAW devices 52are bounded by a number of scribe lines 53 formed of a conductormaterial that may be identical with the material forming the electrodesof the SAW devices. The surface of the substrate 51 is covered by aconductor film 50 except for those regions on which the SAW devices 52are formed, and the scribe lines 53 are connected to the conductor film50. Further, the electrode of each SAW device 52 is connected to theconductor film 50 via a number of conductor strips 55a-55e. It should benoted that FIG. 5 shows the wafer before the dicing is deformed.

FIG. 6 shows the construction of the SAW device according to the firstembodiment. Referring to FIG. 6, the SAW device 52 has a constructionsubstantially identical with the SAW device of FIG. 3 in that a numberof input interdigital electrodes 71 corresponding to the inputinterdigital electrodes 21 and a number of output interdigitalelectrodes 72 corresponding to the output interdigital electrodes 22 areformed on the upper major surface of the piezoelectric wafer 51.

The SAW device is bounded by the scribe line, 53 as shown, and eachinterdigital electrode 71, comprising a first part EL1 corresponding tothe first part EL1 of FIG. 1 and a second part EL2 corresponding to thesecond part EL2 of FIG. 1. The plurality of individual finger electrodesof the first part EL1 are arranged in plural first groups (i.e., twofinger electrodes per group) and are connected commonly to an inputbonding pad 78 on the upper major surface of the wafer 51 and theplurality of individual finger electrodes of the second part EL2 arearranged in plural second groups (i.e., three fingers per group) andeach second group is connected to a respective ground pad 77 provided onthe upper major surface of the wafer 51 in correspondence to theassociated input interdigital electrode 71. Further, each interdigitaloutput electrode 72 comprises a first part EL1' corresponding to thepart EL1 of FIG. 1 and a second part EL2' corresponding to the part EL2of FIG. 1. The plurality of finger electrodes of the first part EL1'likewise are arranged in plural first groups and are connected commonlyto an output bonding pad 78' on the upper major surface of the wafer 51and the plurality of finger electrodes of the second part EL2' likewiseare arranged in plural second groups and each second group is connectedto a respective ground pad 77' provided also on the upper major surfaceof the wafer 51 in correspondence to the associated electrode 72. Asseen in FIG. 6, and with respect to each of the input digital electrode71 (and also as to the output digital electrode 72), each first group ofplural finger electrodes of the first part EL1 (and likewise as to eachfirst group of the first part EL1' of the output digital electrode 72)has respectively associated therewith a second group of plural fingerelectrodes of the second part EL2 of the input digital electrode 71 (andlikewise as to the second group of finger electrodes of the second partEL2' of the output digital electrode 72). Further, these sets ofrespectively associated first and second groups of finger electrodes,respectively electrodes 71 and 72, are provided alternately on the uppermajor surface of the wafer 51 to form a row extending in the propagationdirection of the surface acoustic wave. Thereby, a passage or channel CHof the surface acoustic wave is defined in each SAW device extending inthe X-direction (i.e., in the orientation of FIG. 6) and within a region(extending in the transverse, or Y-direction) wherein the fingers of thefirst part EL1 or EL1' are in an overlapping relationship (i.e., areinterdigitized) with the fingers of the second part EL2 or EL2' whenviewed in and thus relatively to the propagating direction of thesurface acoustic wave. The respective individual finger electrodes ofeach set of associated first and second groups are arranged ininterdigitized, alternating and spaced relationship. Further, the fingerelectrodes of the first group, of the first part EL1 of the inputelectrodes 71, extend in a first direction (downwardly, as seen in FIG.6) transverse to the X-direction of the channel CH, and the individualfinger electrodes of the second group extend in a second direction(upwardly, as seen in FIG. 6) likewise transverse to the X-direction ofthe channel CH. The individual finger electrodes of the first and secondgroups of the output electrode 72 are similarly arranged in alternatingand interdigitized, spaced relationship but extend in the oppositedirections to those of the respective, first and second groups of theinput electrode 71; thus, the finger electrodes of the first groupextend in the second direction and the finger electrodes of the secondgroup extend in the first direction, both being transverse to theX-direction of the channel CH. Further, at the opposite sides of the rowof electrodes 71 and 72, a pair of respective reflectors 75 areprovided, likewise in correspondence to the channel CH.

In the structure of FIG. 6, the input bonding pad 78 is electricallyconnected to the conductor scribe line 53 by the conductor strip 55a.Similarly, the output bonding pad 78' is connected electrically to thescribe line 53 by the conductor strip 55b. Further, the second part EL2and the second part EL2' of the electrodes 71 and 72 are connected witheach other by a pair of interconnection strips 54, and these secondparts EL2 and EL2' are connected to the scribe line 53 via the conductorstrip 55c at the two electrodes 71 located at the outer ends of theelectrode row. Further, each reflector 75 is connected to the part EL2of the adjacent electrodes 71 via the interconnection strip 54 andfurther to the dicing line 53 via the interconnection strip 55e.

As already noted, the scribe lines 53 are formed from a conductor stripand connected to the conductor layer 50 covering the surface of thewafer 51. The conductor layer 50, the scribe lines 53, theinterconnections 54 and 55a-55e, the electrodes 71 and 72, thereflectors 75, and the bonding pads 78 and 78', are all formed fromaluminum-copper alloy sputtered on the wafer 51 with a thickness of 200nm and patterned subsequently by the photolithographic process.

When the SAW device is designed to have the central frequency of 836MHz, the pitch P is set to 2.4 μm while the width W and the separation Sare set to 1.2 μm, as described previously. In the illustrated example,the number of input electrodes 71 is seven while the number of outputelectrodes 72 is six. In each input electrode 71, there are 19 pairs ofopposing electrode fingers with a uniform overlap throughout theelectrodes 71. In each output electrode 72, on the other hand, there are30 pairs of opposing electrode fingers, with a uniform overlapthroughout the electrodes 72. In the reflector 75, there are thirtypairs of fingers. It should be noted that the reflectors 75 have theopen-strip construction wherein the first part EL1 and the second partEL2 are isolated from each other. The concept of an electrode fingerpair and the concept of overlap are defined in FIG. 1. The channel CH isdefined as the region of overlap of the electrode fingers.

Here, it should be noted that the interconnection strips 54 connectingthe second parts EL2 and EL2' of adjacent electrodes 71 and 72 areprovided outside the channel region CH to avoid presenting anydisturbance to the propagating surface acoustic waves. In FIG. 6, thechannel region CH is shown to be included in the region defined betweenthe opposing interconnections 54 to avoid the overlapping of lines inthe drawing. However, it should be noted that the upper and lowerboundaries of the region CH may coincide with the interconnection strips54 as long as the propagation of the surface acoustic wave is notdisturbed. The effect of the interconnection strips 54 being providedwithin the region CH as shown in FIG. 8 will be presented later in acomparison with the SAW device of the present invention as shown inFIGS. 6 and 7. It should be noted that a single interconnection strip 54may be used for connecting the part EL2 and the part EL2', instead ofusing two such strips 54 as illustrated in FIG. 6. The interconnections55a-55e are also provided such that the propagation of the surfaceacoustic waves is not disturbed.

By forming the devices 52 as shown in FIGS. 5 and 6 before the dicingprocess, any positive and negative electric charges that are induced onthe surface of the piezoelectric wafer 51, by the pyroelectricity, areneutralized with each other by flowing through the scribe lines 53, theinterconnections 54 and the interconnections 55a-55e.

After the SAW devices 52 are formed, the SAW devices 52 are separatedfrom each other by dicing along the dicing lines 53 by a diamond sawfixture. Thereby, the devices 52 are mechanically as well aselectrically separated from each other.

FIG. 7 shows a second embodiment of the SAW device of the presentinvention. This embodiment shows the construction of the SAW device of amore practical form. It should be noted, however, that the essentialfeature of the device of FIG. 7 is substantially identical with thedevice of FIG. 6. Thus, those parts corresponding to the parts describedpreviously with reference to FIG. 6 are designated by the same referencenumerals and the description thereof as to FIG. 7 will be omitted.

As shown in FIG. 7, the scribe line 53 is formed with a width d that istypically about 50 μm. After the completion of formation of the SAWdevice on the wafer 51, the devices are separated from each other byperforming the dicing process along the dicing line 53. This dicingprocess is achieved by using a diamond saw that removes the material ofthe wafer 51 over a width D that is larger than the width d. Typically,the width D has a value of 80-100 μm. Thereby, the interconnections55a-55e are separated completely from the ground and hence from thevarious parts, including the finger electrodes of the input and outputelectrodes 71 and 72, that are connected to the interconnections55a-55e. With this process, the SAW device becomes operational. As thefinger electrodes are connected to the ground via the dicing line 53 andthe interconnection strips 55a-55e during the process of forming the SAWdevices 52 on the wafer 51, the problem of accumulation of electriccharges and associated sparking discharge during the heating processemployed in the fabrication of the device is successfully avoided.

According to the present invention, production of the defective devicesdue to the sparking discharge between the electrodes is eliminatedalmost completely. It should be noted that, in the conventional process,50-80% of the devices usually had some form of defects due to thesparking discharge.

FIG. 9 shows the frequency characteristics of the SAW device of FIG. 7.This device has the central frequency thereof set to 835 MHz and isdesigned to have a pass-band between 822.5 MHz and 850.5 MHz. Further,the device is designed to satisfy a specification requiring a side loberejection of -25 dB or more in the frequency range between 869 MHz and894 MHz. In FIG. 9, there is another characteristic curve shown by abroken line. This characteristic curve is for the device of FIG. 8 thatis substantially identical with the device of FIG. 7 except that theregion 4 between each opposing pair of interconnection strips 54 (i.e.,of FIGS. 6 and 7) is filled by aluminum. In other words, the device ofFIG. 8 has an interconnection structure which is arranged to intersectthe propagating path CH of the surface acoustic wave. As can be seen inFIG. 9, the degree of side lobe rejection at 869 MHz is smaller by morethan 5 dB than the device of FIG. 7. Thus, the SAW device of the presentinvention, having the interconnection strips 54 provided so as to avoidthe channel region of the surface acoustic wave, is not only capable ofeliminating the sparking discharge at the time of fabrication but alsoprovides an excellent frequency characteristic.

FIG. 10 shows a third embodiment of the present invention, wherein onlythe cross section is shown. It should be noted that the SAW device ofthe present embodiment of FIG. 10 has a construction identical with thedevice of FIG. 6 or FIG. 7 in the plan view.

Referring to FIG. 10, the side wall and the lower major surface of thewafer 51 are completely covered by conductor layers 50a and 50b. Suchconductor layers may be formed by depositing a titanium film having athickness of about 50 nm and then an aluminum film having a thickness ofabout 300 nm by the vacuum deposition process such that the layers 50aand 50b make an electric connection to the conductor layer 50 on theupper major surface of the wafer 51.

By covering the side wall and the lower major surface of the wafer 51 inthis way, the electric potential at the surface of the wafer 51 becomesidentical throughout the entire surface and the problem of sparkingdischarge of the finger electrodes due to the pyroelectricity iscompletely eliminated. After the structure of FIG. 10 is formed, the SAWdevices 52 are formed by a series of photolithographic patterningprocesses that are well established in the art.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention. corresponding toa surface-acoustic-wave device said first conductor strip beingconnected with each device pattern electrically, each of said devicepatterns including a plurality of interdigital electrodes provided onthe upper major surface of the substrate so as to form a row incorrespondence to a passage of surface acoustic waves such that theelectrodes are aligned, as a row, in the propagating direction of thesurface acoustic waves, each of said plurality of interdigitalelectrodes comprising a first part connected to a bonding pad providedon the upper major surface of the substrate for external electricconnection and having a plurality of finger electrodes extendingparallel with each other in a direction that is included in the uppermajor surface of the substrate and crossing the passage of the surfaceacoustic waves, and a second part separated from the first part andhaving a plurality of finger electrodes extending parallel with eachother in a direction opposite to the direction of the finger electrodesof the first part, said first part and said second part being sodisposed, in each interdigital electrode, that said finger electrodes ofthe first part and said finger electrodes of the second part arerepeated alternately in the propagating direction of the surfaceacoustic waves with an overlapping of the opposing finger electrodeswhen viewed in the propagating direction of the surface acoustic waves,said opposing finger electrodes defining the passage of the surfaceacoustic waves, a second conductor strip provided on the upper majorsurface of the substrate in correspondence to a region offset from thepassage of the surface acoustic waves for connecting the second parts ofadjacent interdigital electrodes with each other, a third conductorstrip provided on the upper major surface of the substrate incorrespondence to a region offset from the passage of the surfaceacoustic waves for connecting the bonding pad to the first con

What is claimed is:
 1. A method for fabricating a surface-acoustic-wavedevice, comprising the steps:covering an upper major surface of apiezoelectric substrate with a conductor layer; patterning the conductorlayer to form a plurality of device patterns that are separated fromeach other by a first conductor strip, said first conductor strip beingformed so as to extend along a dicing line defined on the upper majorsurface of the substrate for separating the substrate into a number ofchips each ductor strip electrically, and a fourth conductor stripprovided on the upper major surface of the substrate in correspondenceto a region offset from the passage of the surface acoustic waves forconnecting the second part of the interdigital electrodes to the firstconductor strip and; and separating the surface-acoustic-wave devicesfrom each other by dicing along the dicing line that the first conductorstrip is removed from each surface-acoustic-wave device.
 2. A method asclaimed in claim 1 in which said step of separating thesurface-acoustic-wave devices comprises a sawing process wherein thesubstrate is cut along the dicing line by a saw with a widthsubstantially larger than a width of the first conductor strip.
 3. Amethod as claimed in claim 1 in which each of said plurality of devicepatterns further comprises a pair of reflector patterns provided at bothends of the row of electrodes for reflecting the surface acoustic waves,wherein each of said reflector patterns is connected to the firstconductor strip.
 4. A method as claimed in claim 2 in which said step ofpatterning the conductor layer is performed such that the conductorlayer is left on the upper major surface of the substrate and surroundsthe plurality of device patterns, said patterning being made such thatthe first conductor strip is connected to the conductor layer left onthe upper major surface of the substrate.
 5. A method as claimed inclaim 4 in which said step of covering the upper major surface of thesubstrate is carried out such that a side wall and a lower major surfaceof the substrate opposing the upper major surface of the substrate arecovered by a conductor layer such that the conductor layer covering theside wall and such that the lower major surface of the substrate isconnected electrically to the conductor layer left on the upper majorsurface of the substrate, and said step of patterning is performed suchthat the conductor layer covering the side wall and the lower majorsurface of the substrate is left unpatterned.